1. Field of the Invention
The present invention relates to a solid-state imaging device and a method of manufacturing such a solid-state imaging device, and more particularly to a solid-state imaging device having a film disposed on a sensor thereof for suppressing light reflections from a semiconductor substrate, and a method of manufacturing such a solid-state imaging device.
2. Description of the Related Art
Conventional solid-state imaging devices are disclosed in Japanese Patent Laid-Open Publication No. 206571/92 and Japanese Patent Laid-Open Publication No. 152674/92, for example. The disclosed solid-state imaging devices have a reflection-resistant film disposed on the entrance surface of a photodiode for increased sensitivity thereof.
The solid-state imaging device disclosed in Japanese Patent Laid-Open Publication No. 206571/92 is shown in FIG. 1 of the accompanying drawings. As shown in FIG. 1, the solid-state imaging device has a photodiode 55 as a sensor and a vertical CCD (Charge-Coupled Device) 56 which are disposed in a surface layer of a silicon substrate 59. A silicon oxide film 49 serving as an oxide film is disposed over the photodiode 55 and the vertical CCD 56. An intermediate-refractive-index film 48 is disposed on the surface of the silicon oxide film 49. The silicon oxide film 49 and the intermediate-refractive-index film 48 are shared by the photodiode 55 and the vertical CCD 56.
Gate electrodes 46 are disposed on the surface of the intermediate-refractive-index film 48 over the vertical CCD 56. The gate electrodes 46 are covered with light blocking films 44 of aluminum (Al) or tungsten (W) with interlayer insulating films 45 interposed therebetween. The surfaces of the light blocking films 44 and the surface of the intermediate-refractive-index film 48 over the photodiode 55 are covered with a protective film 43 such as a silicon oxide film. The intermediate-refractive-index film 48 comprises a silicon nitride film. The silicon nitride film as the intermediate-refractive-index film 48 has a refractive index which is of a substantially intermediate value between the refractive index of the silicon oxide film as the protective film 43 and the refractive index of the silicon substrate 59. The refractive index of the intermediate-refractive-index film 48 is smaller than the refractive index of the silicon substrate 59, and the refractive index of the protective film 43 is smaller than the refractive index of the intermediate-refractive-index film 48. A planarizing film 42 is disposed on the surface of the protective film 43 for planarizing surface irregularities of the protective film 43.
FIG. 2 of the accompanying drawings shows the solid-state imaging device disclosed in Japanese Patent Laid-Open Publication No. 152674/92. As shown in FIG. 2, the solid-state imaging device has a photodiode 75 and a vertical CCD 76 which are disposed in a surface layer of a silicon substrate 79 that is covered with a gate film 70. Gate electrodes 66 are disposed on the gate film 70 over the vertical CCD 76. An intermediate-refractive-index film 68 is disposed over surfaces and sides of ends of the gate electrodes 66 and the surface of the photodiode 75 with an insulating film interposed therebetween.
Light blocking films 64 are disposed over the surfaces of the gate electrodes 66 and the surfaces of the intermediate-refractive-index film 68 superposed on the ends of the gate electrodes 66 with an interlayer insulating film 65 interposed therebetween. The surfaces of the light blocking films 64 and the surface of the interlayer insulating film 65 over the photodiode 75 are covered with a protective film 63. The intermediate-refractive-index film 68 has a refractive index which is of a substantially intermediate value between the refractive index of a silicon oxide film used as the interlayer insulating film 65 and the protective film 63 and the refractive index of the silicon substrate 79. The refractive index of the intermediate-refractive-index film 68 is smaller than the refractive index of the silicon substrate 79, and the refractive index of the interlayer insulating film 65 and the protective film 63 is smaller than the refractive index of the intermediate-refractive-index film 68.
In the solid-state imaging device shown in FIG. 2, the ends of the intermediate-refractive-index film 68 overlap the ends of the gate electrodes 66. The thickness of the film between the sides of the ends of the gate electrodes 66 and the intermediate-refractive-index film 68 is greater than the thickness of the film between the intermediate-refractive-index film 68 and the photodiode 75. These dimensional features are effective to reduce any smearing of image signals generated by the solid-state imaging device.
In both the conventional solid-state imaging devices shown in FIGS. 1 and 2, the silicon substrate having the photodiode and a thin film such as the silicon oxide film on the photodiode have largely different refractive indices. In order to minimize light reflections from a boundary surface between the silicon substrate and the thin film, the intermediate-refractive-index film is used as a reflection-resistant film. As described above, the refractive index of the intermediate-refractive-index film is of a substantially intermediate value between the refractive index of the thin film such as the silicon oxide film on the silicon substrate and the refractive index of the silicon substrate. The intermediate-refractive-index film which is disposed on the surface of the silicon substrate either directly or with the insulating film interposed therebetween is effective to suppress light reflections from the semiconductor substrate. As a result, the sensitivity of the solid-state imaging device is increased.
With the solid-state imaging device disclosed in Japanese Patent Laid-Open Publication No. 206571/92, the intermediate-refractive-index film which covers the entire surface of the semiconductor substrate comprises a silicon nitride film which has a low hydrogen permeability. The intermediate-refractive-index film of a low hydrogen permeability which covers the entire surface of the semiconductor substrate fails to provide a hydrogen alloying effect. Hydrogen alloying occurs to reduce oxygen in the semiconductor substrate with hydrogen and remove the reduced oxygen from the semiconductor substrate. Upon hydrogen alloying, hydrogen passes through the silicon oxide film and tungsten. However, since hydrogen is blocked by the silicon nitride film as the intermediate-refractive-index film and does not reach the semiconductor substrate, no hydrogen alloying effect takes place. As a result, the solid-state imaging device produces an increased dark current.
The solid-state imaging device disclosed in Japanese Patent Laid-Open Publication No. 152674/92 has intermediate-refractive-index films associated with respective photodiodes. In the fabrication of the disclosed solid-state imaging device, it is necessary to deposit an intermediate-refractive-index film over the entire surface of the semiconductor substrate and thereafter etch away unwanted areas of the deposited intermediate-refractive-index film. Consequently, the process of fabricating the disclosed solid-state imaging device needs an additional step of etching away unwanted areas of the deposited intermediate-refractive-index film.